Neurosecretory peptides are important functionally in the central nervous system both at the behavioral and cellular levels. For model studies the invertebrate nervous system offers distinct advantages over the vertebrate preparation since it allows rigorous investigation of peptide biochemistry and physiology at the single cell level. An identified cluster of neurosecretory cells, the Bag Cell neurons in the parietovisceral ganglion of the sea mollusk Aplysia californica, release neurohormonal factor(s) which both stimulate(s) egg-laying in mature Aplysia and modify existing electrical activity of specific neurosecretory cells. Experiments are planned to (1) purify, sequence and synthesize the hormonal factor(s) elaborated by the Bag Cells and (2) to explore the biosynthesis and precursor-product relationships of peptides in these cells. Functional studies of hormonal factors isolated from Aplysia will be carried out on Aplysia, as well as on land snails (Otala lactea and Helix aspersa) and fresh water snails (Lymnaea stagnalis), each of which provides distinct assay advantages. Possible evolutionary, functional and structural relationships between Bag Cell peptide factors and the vertebrate neurohypophyseal hormones and their carrier proteins will be evaluated.